1. Field of the Invention
The present invention relates to a brake control circuit, an elevator system as defined in the preamble of claim 9, and also a method for controlling the brake of the elevator system.
2. Background of the Invention
It is very general to use a machinery brake that mechanically connects with a rotating part of the elevator machine as a braking apparatus of an elevator car. The machinery brake can be in its structure e.g. a drum brake or a disc brake. The braking function of a machinery brake is conventionally activated by disconnecting the electricity supply circuit of the brake control winding, e.g. with a relay or contactor. After the electricity supply of the brake has been disconnected the brake closes, in which case brake pad attached to the brake shoe connects mechanically with a rotating part of the machine. The closing of the brake occurs with a closing delay, which is determined from the electrical parameters of the brake and of a possible attenuation circuit, such as from the inductance and resistance of the brake, as well as from the impedance of the possible attenuation circuit.
The force exerted by a brake is generally quite large, so that when activating the braking function e.g. in connection with an emergency stop, the brake pad engages to brake the movement of the elevator car with the kind of deceleration of movement that might feel uncomfortable to a passenger in the elevator car.
Rather a lot of kinetic energy is also generated when the brake operates. This produces a loud noise when the brake pad hits against the braking surface. To solve this problem the aim has been for the distance between the brake pad and the braking surface to be as small as possible. In this case the brake pad does not have time to achieve a very great speed and kinetic energy when it hits closed, as a result of which the impact is more subdued. An air gap that is small enough is, however, difficult to implement and also to adjust, and this type of solution results in a very fragile structure and also in extremely precise manufacturing tolerances.
The operation of a brake of an elevator can be affected also by adjusting the current of the brake. Publication JP 2008120521 presents one such type of adjustment of the brake current wherein the braking force is measured from the brake drum with a special pressure sensor, and the current of the excitation winding of the brake is adjusted on the basis of the measuring signal of the pressure sensor. In this case the braking force can be affected with the adjustment of the brake current.
Publication JP 2008120469 presents an arrangement wherein it is endeavored to reduce the noise produced by the operation of a brake by changing the impedance of the electricity supply circuit of the brake in stages such that the change in impedance also affects the magnitude of the brake current.